Membranes and their manufacture

ABSTRACT

Membranes comprising an aromatic ether ketone polymer (e.g. polyetherketone) in sheet or hollow fibre form are made by preparing at least a 12 wt % solution of the polymer in a solvent which is chemically inert to the polymer (e.g. concentrated sulphuric acid); forming a film of the solution in sheet or hollow form; and contacting the film with a non-solvent (e.g. more dilute sulphuric acid or acetic acid) thereby to precipitate the film.

[0001] This invention relates to membranes, particularly membranessuitable for use in the separation of one gas from another. Theinvention relates in particular to membranes comprising an aromaticether ketone polymer.

[0002] Membranes of aromatic ether ketone polymers are known. EP 382356Adescribes such membranes, and their use in ultrafiltration andmicrofiltration. This specification states that when such membranes aresuitable for ultrafiltration they may be used as the porous substrate ofcomposite membranes for use in gas separation, pervaporation and reverseosmosis membranes.

[0003] The manufacture of the membranes of EP 382356A involves thedissolution of the selected aromatic ether ketone polymer in a strongacid, casting the solution, then precipitating the polymer by treating afilm of the solution, in the required shape, with a non-solvent.Examples of strong acids suitable as solvents are methanesulphonic acid,fluoroalkane sulphonic acids, liquid hydrogen fluoride and sulphuricacid. Examples of suitable non-solvents are acetic acid, dilutesulphuric acid, water, methanol and ethanol.

[0004]FIG. 6 of EP 382356A is a photomicrograph of a membrane produced.It shows a region of very large “finger” voids, beneath a region ofcellular voids (a so-called “sponge” structure). Such finger voids arevery undesirable for gas separation uses.

[0005] The membranes produced by the method of EP 382356A are apparentlyunsuitable for using as such, in gas separation. As already noted it isstated that they could be used for this purpose, but in compositemembranes. Example 44 describes the production of a gas separationmembrane. An aromatic polyether ketone membrane is made by the methoddescribed above and is then brush coated with liquid silicone rubber,and the silicone cured for 16 hours at room temperature.

[0006] In EP 499381A similar technology is described, with thedifference that the hollow fibre membrane produced may be drawn. This issaid to yield the benefit that during filtration the flux through themembrane is increased but the molecular weight cut-off is unchanged.

[0007] Gas separation is an important field, but the technology of EP382356A and EP 499381A appears unpromising for this purpose, in that themembranes they disclose require an additional coating step, before theycan be used for gas separation. Not only does this additional step meanadditional manufacturing complexity and additional cost, it also meansthat the membrane has two distinct materials, each with differentdiffusion constants for the gases being separated. This is anundesirable situation.

[0008] It has now been discovered, surprisingly, that the general methoddescribed in EP 382356A and EP 499381A may, in fact, be a usefulstarting point for the more effective manufacture of membranes, withproperties making them suitable for use as gas separation membranes.

[0009] In accordance with a first aspect of the present invention thereis provided a method of making a membrane of sheet or hollow fibre form,the membrane comprising an aromatic ether ketone polymer, the methodcomprising the steps of:

[0010] a) preparing a solution of the polymer in a solvent which ischemically inert to the polymer, the concentration of the polymer in thesolution being at least 12%;

[0011] b) forming a film of the solution in sheet or hollow fibre form;and

[0012] c) contacting the film with sulphuric acid of concentration inthe range 50-80%, thereby precipitating the membrane of said polymer.

[0013] In accordance with a second aspect of the present invention thereis provided a method of making a membrane of sheet or hollow fibre form,the membrane comprising an aromatic ether ketone polymer, the methodcomprising the steps of:

[0014] a) preparing a solution of the polymer in a solvent which ischemically inert to the polymer;

[0015] b) forming a film of the solution in sheet or hollow fibre form;and

[0016] c) contacting the film with a non-solvent, thereby precipitatingthe membrane of said polymer, the interval between the addition of thenon-solvent and the completion of the precipitation being at least 5minutes, preferably at least 10 minutes.

[0017] In accordance with a third aspect of the present invention thereis provided a method of making a membrane of sheet or hollow fibre form,the membrane comprising an aromatic ether ketone polymer, the methodcomprising the steps of:

[0018] a) preparing a solution of the polymer in a solvent which ischemically inert to the polymer;

[0019] b) forming a film of the solution in sheet or hollow fibre form;and

[0020] c) contacting the film with a non-solvent, thereby precipitatingthe membrane of said polymer;

[0021] wherein the solvent and non-solvent are selected such that whenthey are mixed the temperature rises by not more than 30° C. (whentested as described hereinafter).

[0022] The method of the second aspect may be used together with themethod of the first aspect.

[0023] The method of the second aspect may be used in conjunction withthe method of the third aspect.

[0024] The method of the first aspect may be used in conjunction withthe method of the third aspect.

[0025] The methods of the first, second and third aspects may be usedtogether.

[0026] The following definitions of preferred features and conditionsmay be read in conjunction with the first and/or second and/or thirdaspect(s) of the invention.

[0027] Preferably the polymer comprises an at least partiallycrystalline aromatic ether ketone polymer.

[0028] By partially crystalline we mean that the level of crystallinityis at least about 5%, preferably at least 10%. Such crystallinity ismeasured by wide angle X-ray diffraction as described by Blundell andOsborn (Polymer 24, 953, 1983).

[0029] By “aromatic ether ketone polymer” we mean a polymer in whichinter-ring ether linkages and inter-ring ketone linkages togetherprovide at least a major portion of the linkages between aromatic unitsin the polymer backbone. We do not exclude the possibility that aportion of the aromatic rings may be replaced by heterocyclic rings, forexample pyridine.

[0030] As examples of aromatic ether ketone polymers of which themembrane according to the present invention may be comprised may bementioned inter alia the polymers and copolymers illustrated in FIGS. 1to 4.

[0031]FIG. 1 illustrates polymer chains in which the aromatic rings arejoined by ether or ketone bonds (I-VI);

[0032]FIG. 2 illustrates polymer chains in which a portion of thearomatic rings are joined by direct links (VII-IX) or are bicyclic ring(X);

[0033]FIG. 3 illustrates copolymer units bearing intercyclic —SO₂— bonds(XI-XII);

[0034]FIG. 4 illustrates certain copolymers containing ketone and etherlinks (XV) or in addition, a mixture of biphenyl and sulphone linkages(XIV and XVI).

[0035] Some polymer types are hereinafter referred to for convenience bythe trivial name appended thereto in the drawings.

[0036] It will be appreciated that in FIGS. 1-4,

[0037] E represents an ether linkage;

[0038] K represents a ketone linkage;

[0039] D represents a direct linkage;

[0040] m represents a meta substituted aromatic ring;

[0041] N represents a naphthalene ring; and

[0042] S represents a sulphone linkage except where it is used as aprefix to the polymer trivial name where it represents “sulphonated”.

[0043] We do not exclude the possibility that at least a portion of theether linkages in the polymers illustrated in FIGS. 1-4 may be replacedby thioether linkages.

[0044] The preparation of polymers illustrated in certain figures of thedrawings are described in inter alia the Journal of MacromolecularScience, Review of Macromolecular Chem.

[0045] Phys., (27 (2), 313-341, 1987 (General Formulae I-VIII); Europeanpatent specification No. 0323076 (General Formula IX); Polymer 1984,vol. 25 (August), 1151 (General Formula X); EPA 0194062 (General FormulaXIV) and British patent application BPA 89 10549 (General Formula XVI).

[0046] Whereas the polymer is preferably a homopolymer, egpolyetheretherketone or PEK we do not exclude the possibility that itmay be a copolymer eg polyetheretherketone/PEK,polyetheretherketone/PES, PEK/PES, polyetheretherketone/PEES, whereinthe copolymer units are represented by the General Formula XI-XII in thedrawings appended hereto.

[0047] Preferred polymers for use in the present invention are thosewhich are resistant to sulphonation in concentrated sulphuric acid,notably PEKK, PEKEKK and, especially PEK.

[0048] It will be appreciated that when a copolymer is used the natureand molar percentage of the “comonomer” will be chosen such that it doesnot unduly decrease the crystallinity of the polyetherketone componentof the copolymer and that the inter-aromatic links therein arenon-hydrolysable under the conditions of preparation and expected use ofthe membrane. As examples of comonomer units may be mentioned inter aliaaromatic sulphones, sulphides, and biaryls.

[0049] Preferably the solvent is a strong acid. Preferably it is asubstantially non-sulphonating reagent towards the aromatic ether ketonepolymer. For example, where the aromatic ether ketone polymer ispolyetheretherketone (or another aromatic ether ketone polymer havingO-Ph-O— or other electron-rich readily sulphonatable units) the strongacid is typically a sulphonic acid, for example methanesulphonic acid,rather than sulphuric acid.

[0050] The solution is formed by dissolving the polymer, typically inparticulate form, in the strong acid under an inert atmosphere, at atemperature and timespan sufficient to completely dissolve the polymer.Preferably no pore-forming agent is present.

[0051] A sheet membrane is typically formed by casting the polymersolution as a thin film, typically of thickness between 20 and 500 μm,onto a suitable substrate which is not attacked by the solution. Themembrane can be supported by a porous fabric or film eg polyethylene,polypropylene, polyetheretherketone, polyester, PTFE or carbon fibre.Alternatively, the membrane can be unsupported in which case the filmwould be cast onto a plane non-porous surface, eg a band of stainlesssteel, PTFE or polypropylene or a sheet of glass. Of course, the skilledperson must select a material which is not degraded by the solutionbeing cast onto it.

[0052] The polymer is then precipitated by treating the shaped solutionwith the non-solvent under suitable conditions.

[0053] For example it may be immersed in non-solvent liquid in agelation bath, or non-solvent vapour may be allowed or caused to diffuseinto it.

[0054] A fibre capillary membrane may be formed by extruding the polymersolution through the outer annulus of a coaxial die. Through the innernozzle there is a flow of suitable fluid, eg an inert gas or liquid,which is a non-solvent for the polymer. Detail on the preparation ofsuch membranes is provided in EP 0499 381 A1 and the content thereof isincorporated herein by reference.

[0055] The precipitated membrane is then allowed to remain in contactwith the non-solvent for a time sufficient to allow substantiallygelation of the polymer, then removed from the non-solvent.

[0056] The residual solvent/non-solvent is removed from the membrane bytreatment with an aqueous medium, eg water or an aqueous base, at anappropriate temperature, eg between room temperature and the boilingpoint of water. Further treatment with an organic medium may benecessary to reduce the amount of acid present in the membrane further.

[0057] The membrane may be subjected to a subsequent treatment toenhance crystallinity. As examples of such treatment may be mentionedinter alia heating the membrane, preferably in a substantially drystate, and preferably above the Tg of the polymer, under an inertatmosphere; and treatment with a polar aprotic solvent, for exampleacetone, dimethyacetamide (DMA), dimethylformamide (DMF),tetrahydrofuran (THF) and dichloromethane. After use of a crystallinityincreasing solvent, especially acetone, dichloromethane ortetrahydrofuran, there is preferably a further step which involvesremoval of the solvent, for example by evaporation.

[0058] Preferably the solvent is substantially water-free although we donot exclude the possibility that it may contain a small amount, forexample up to 10%, of water.

[0059] It will be appreciated that the solvent will be chosen in thelight of the structure of the polymer. For example, it should not reactchemically with the polymer or unduly reduce the crystallinity thereof,on precipitation. For example, whereas 98% sulphuric acid reacts withpolyetheretherketone and should not be used therewith, it canadvantageously be used with polyetherketone (PEK) in the processaccording to the present invention. The solvent should be a good solventfor the polymer but inert to it. After membrane formation it should bereadily extractable therefrom.

[0060] We do not exclude the possibility that a preferred strong acidsolvent may be a mixture of acids, eg sulphuric acid and acetic acid. Itwill be appreciated that where one of the acids in the mixture is anon-solvent for the polymer the concentration thereof will beinsufficient to inhibit solvation of the polymer in the mixture. Onepossible strong acid solvent is a mixture of sulphuric acid and aceticacid, the concentration of acetic acid typically being less than 15%w/w.

[0061] As examples of strong acids for use in the process of the presentinvention may be mentioned inter alia sulphuric acid, liquid hydrogenfluoride, methane sulphonic acid, fluoromethane sulphonic acid, and di-and tri-fluoromethane sulphonic acid.

[0062] It will be appreciated that the skilled person will takeappropriate precautions when using any of the above acids.

[0063] We do not exclude the possibility that a further solvent may beused in combination with the strong acid, with the proviso that it doesnot unduly impair the solvent power of the strong acid or unduly reactwith the polymer in the presence of the strong acid.

[0064] As typical examples of such further solvents may be mentionedinter alia liquid sulphur dioxide, 1,2,4-trichlorobenzene,1,2-dichloroethane, dichloromethane, dichloroacetic acid andtrifluoroacetic acid.

[0065] Typically the concentration of the polymer in the solvent is atleast 12%, preferably at least 14%, most preferably at least 16%. Theupper limit may be determined empirically. It may, for example, bedetermined by viscosity considerations or by the ultimate upper limit ofthe concentration of the polymer in the solvent. However in typicalpractical examples the concentration is up to 40%, preferably up to 30%,most preferably up to 25%.

[0066] As examples of non-solvents used in the process of the presentinvention may be mentioned inter alia polyhydric alcohols, concentratedacetic acid, and somewhat dilute sulphuric acid. The non-solvent used inthe process of the present invention may be a single component or amixture. However in preferred methods there is a single solvent.

[0067] A preferred solvent, when the polymer is resistant tosulphonation by it, is concentrated sulphuric acid, preferably of atleast 96% concentration.

[0068] When concentrated acetic acid is used it is suitably ofconcentration at least 80%, preferably at least 90%.

[0069] When a polyhydric alcohol is used in the present invention it ispreferably used neat.

[0070] When somewhat dilute sulphuric acid is used as a non-solvent inthe present invention it is suitably of concentration at least 40%,preferably at least 50%, more preferably at least 55%, and mostpreferably at least 60%. Preferably dilute sulphuric acid is ofconcentration not exceeding 80%, and most preferably not exceeding 72%.

[0071] Preferably the mixing of the non-solvent with the solution isaccompanied by a temperature rise not exceeding 27° C., and preferablynot exceeding 25° C. (when tested as hereinafter described).

[0072] A preferred method of the invention is for making a sheet orhollow fibre membrane of an at least partially crystalline aromaticether ketone polymer comprising O-phenyl-C(O)— units and substantiallyfree of —O-phenyl-O— or other electron-rich readily sulphonatable units,the method comprising the steps of

[0073] a) preparing a solution of the polymer in concentrated sulphuricacid of concentration at least 90%, the concentration of the polymer inthe solution being in the range 14-30%, preferably 16-25%;

[0074] b) forming a film of the solution in sheet or hollow fibre shape;and

[0075] c) contacting the film with sulphuric acid of strength in therange 50-80%, preferably 55-72%, thereby precipitating the sheet orhollow membrane of the polymer.

[0076] The measures described herein are significant in promoting animproved membrane, suitable for use in gas separation.

[0077] Preferred membranes produced by the method of the presentinvention have, in cross-section, a surface layer, preferablysubstantially void-free, and a base layer which is wholly orpredominantly of cellular structure, and substantially free of, or withno, finger voids. The measures of the present invention seek to maximisethe cellular or “sponge” structure and minimise, or preferablyeliminate, the finger voids. Where there are voids present (other thanthe cellular microvoids of the “sponge” structure) they suitably do notcompromise gas separation performance.

[0078] In accordance with a fourth aspect of the present invention thereis provided a membrane of a partially crystalline aromatic ether ketonepolymer, the membrane being of sheet or hollow fibre form, the membranecomprising, in cross-section, a surface layer, preferably substantiallyvoid-free, and a base layer which is wholly or predominantly of “sponge”cellular void structure. Preferably such a membrane has substantially nofinger voids. Preferably it contains substantially no large voids whichextend within the base layer as far as the surface layer. Preferably itcontains no large voids which extend within the base layer from theunder-surface of the base layer (that is, opposite to the surfacelayer). It is especially preferred that it contains no largethrough-voids which extend through the entire thickness of the baselayer. It is especially preferred that it has substantially no fingervoids.

[0079] Suitably, in use, the membrane of the fourth aspect of thepresent invention is used as a gas separator as such, without beingcoated by or laminated to a different material. Preferably the membraneitself is able to selectively pass gas A, of a gas mixture A+B.

[0080] In accordance with a fifth aspect of the present invention thereis provided gas separation apparatus comprising a membrane made by amethod of the first, second or third aspect of the present invention, orcomprising a membrane of the fourth aspect.

[0081] In a preferred embodiment there is provided a method of preparinga polyetherketone membrane, preferably for gas separation, the methodincluding the steps of:

[0082] preparing a solution (preferably of concentration between 15 and20%) of polyetherketone in concentrated sulphuric acid (preferably 98%H₂SO₄);

[0083] casting the solution;

[0084] causing phase inversion by use of a non-solvent selected fromdiluted sulphuric acid, glycerol and acetic acid. A preferred sulphuricacid concentration is 50-60% H₂SO₄; a preferred glycerol concentrationis 30-40% glycerol; and a preferred acetic acid concentration is 70-90%acetic acid.

[0085] The invention is further illustrated by reference to thefollowing Examples, in conjunction with the drawings, in which:

[0086]FIGS. 1-4 show structures of aromatic polyether ketones;

[0087]FIGS. 5-8 are photomicrographs of PEK membranes prepared frompolymer solutions of different strengths, with water as non-solvent;

[0088]FIGS. 9 and 10 are photomicrographs of PEK membranes preparedrespectively using methanol and ethanol as non-solvents;

[0089]FIGS. 11-13 are photomicrographs of PEK membranes prepared usingacetic acid of different concentrations, as non-solvent;

[0090]FIGS. 14-16 are photomicrographs of PEK membranes prepared usingsulphuric acid of different concentrations, as non-solvent;

[0091]FIGS. 17 and 18 are graphs showing the relationship betweenconcentration of sulphuric acid used as non-solvent, and theprecipitation time and the temperature increase on mixing; and

[0092]FIG. 19 is a graph summarising the results obtained usingdifferent non-solvents.

[0093] Membrane Manufacture

[0094] The polymer used for membrane manufacture was polyetherketone(PEK), grade, MV0.392, from Victrex, of Blackpool, UK. The polymer wasdried for 24 hours at 100° C. under vacuum prior to dissolution. Thesolvent (98% sulphuric acid, A. R. grade) was first placed into areactor vessel, and then heated to 50° C. in a water bath. The requiredamount of PEK polymer was then added to the solvent. The polymer solventmixture was stirred between 300 to 500 rpm for about 5 hours until aviscous red solution of PEK in 98% sulphuric acid was obtained.

[0095] The film casting procedure was developed to evaluate particularcombinations of polymer, solvent and non-solvent. The polymer solutionwas cast at room temperature on to a glass plate (70 mm×110 mm×4 mm) anda draw down technique was used to produce films of wet thickness 250 μm.After a few seconds, the plate was immersed in a vessel with the desirednon-solvent. After the precipitation process the films separated fromthe glass plate and then they were removed from the vessel. The filmswere then placed in a water bath (in which the water was changed twicedaily) and left for one week to remove any traces of residual solvent.

[0096] Precipitation Time

[0097] The term “precipitation time” was defined to describequalitatively the phase inversion process. The precipitation time wasrecorded as the time period from when the nascent cast film (with wetthickness 250 μm) was immersed into the vessel, to when the membraneseparated completely from the glass plate. In other words the shorterthe “precipitation time”, the more rapidly the polymer precipitates inthe non-sol-vent and separates from the plate.

[0098] Determination of the ΔT of Mixing of Solvent and Non-SolventSystems

[0099] A simple method was devised to determine the temperature rise onmixing the solvent and non-solvent system. The solvent (total 5 ml) wasadded at a rate of 1 ml/min via a burette into 20 ml of non-solvent(with slow stirring) at room temperature. After all the solvent had beenadded any temperature variation that accompanied the mixing of thesolvent and non-solvent was measured using a thermometer to +/−0.5° C.

[0100] Examination of Membrane Morphology

[0101] The cross-sectional membrane morphologies were observed usingScanning Electron Microscopy (SEM) (Cambridge S360). The fracturesurfaces were produced by breaking the membranes in liquid nitrogen andsputtered with gold.

[0102] Effect of Polymer Concentration on Membrane Structure

[0103] The scanning electron micrographs shown in FIGS. 5-8 arecross-sections of membranes produced from PEK polymer solutions in 98%sulphuric acid. In each case water (at 20° C.) was used as thenon-solvent. It is clear that at different polymer concentrations, i.e.,10%, 15%, 18% and 20%, different morphologies are produced. In each casethe top of the respective film, that is the side that was never incontact with the glass casting plate is at the top of micrograph.

[0104] Films cast from 10% PEK polymer solutions show large disruptivemacro-voids (including finger voids) in the cross-section. Pores on thetop surface of film were visible using the SEM. In addition to this, thethickness of the dense skin (the top separation layer) was very smalland overall the structure would not provide sufficient self-supportingmechanical properties.

[0105] Films cast from 15% and 18% PEK show regular finger voids in thecentral part of cross-section. No pores on the top surface were visibleusing the SEM. It can be seen that a significant number of finger voidspenetrate through the whole film and channels can be observed that leadfrom the top to the bottom of the membrane.

[0106] The films cast from 20% PEK solutions have a thick denseseparating layer. Finger voids penetrated the whole length of thecross-section and these were observed to be narrower and more needleshaped than those film cast from lower polymer concentrations. A poorlydeveloped cellular sponge structure was also observed.

[0107] The precipitation times for these membranes was as follows:

[0108] 10% PEK: 32.9 seconds

[0109] 15% PEK: 60.2 seconds

[0110] 18% PEK: 80.2 seconds

[0111] 20% PEK: 360 seconds

[0112] Although none are of use in the present invention theseexperiments suggest that better results may be achieved with longerprecipitation times.

[0113] Effect of Non-Solvents on Membrane Structure

[0114] The non-solvents used in phase inversion processes greatly affectthe gas permeation properties of membranes. Studies of the PEKpolymer/solvent/non-solvent system provided information for the choiceof suitable non-solvents. Precipitation times corresponded to differentliquid-liquid demixing processes and resulted in a variety of membranestructures.

[0115] In this study, water, methanol, ethanol, and weak acids wereshown to act as non-solvents by precipitating 20% concentration PEK from98% sulphuric acid. Using different kinds of non-solvent a variety offilm morphologies were obtained as shown in FIGS. 9-13. Differentmorphological zones are apparent in the top skin and in the bottommembrane layers. Changing from water as non-solvent (FIG. 5) to one ofhigh acetic acid concentration reduced the amount of finger voids in thestructure. Acetic acid based coagulants aid in the formation ofsponge-like structures and produce reasonable morphologies for gasseparation. Table 1 below shows the membrane precipitation times and ΔTof mixing.

[0116] If methanol and ethanol were used as non-solvents they producedsimilar PEK film morphologies. These membranes have high void volumesand overall the structures are characterised by large macro-voids acrossthe film sections. The final structures were similar to those ofmembranes cast from low polymer concentrations when water was used asthe non-solvent, and are not of use. TABLE 1 Non-solvents -precipitation time of 20% PEK in 98% sulphuric acid and ΔT on mixingPrecipitation ΔT of mixing Non-solvents time (sec.) (° C.) Water 80 55Methanol 120 51 Ethanol 278 47 30% Acetic acid 150 53 70% Acetic acid220 39 90% Acetic acid 966 30

[0117] Use of Sulphuric Acid as Non-Solvent

[0118] In an attempt to obtain better sponge-like substructures in PEKmembranes moderately dilute sulphuric acid was studied as a possiblenon-solvent. A number of solutions, 30%, 50% and 60% of sulphuric acidwere used and membranes precipitated from films cast from 18% PEK in 98%sulphuric acid gave improved morphological structures as shown in FIG.7.

[0119] Films cast using 30% sulphuric acid as the non-solvent exhibitedundesirable finger voids (FIG. 14). Films cast using 50% and 60%sulphuric acid had well-developed interconnected cellular structuresthat ran from the top to the bottom of the membrane (FIGS. 15, 16).These structures were not entirely macrovoid-free, but a highly reducednumber of macrovoids were observed, and they were of rounded “teardrop”shape, rather than being of finger shape. These were the most favourablemorpholgies obtained, for gas separation.

[0120] The more concentrated-sulphuric acid materials thus gave bettermembrane morphology. They also gave longer precipitation times and lowerΔT on mixing, as can be seen in FIGS. 17 and 18 respectively.

[0121] Conclusion and Summary

[0122] Non-solvents which gave lower ΔT values and longer precipitationtimes gave better membranes for gas separation. The relationship betweentemperature rise (heat of mixing solvent and non-solvent), precipitationtime and membrane structure is shown in the overall summary graph, FIG.19. Decreased heat of mixing appears to favour desirable sponge-typecellular structures whilst high heat of mixing appears to favourundesirable finger void structures.

[0123] In this specification all % concentration values are expressed aswt/wt (for example 50% sulphuric acid means 50% sulphuric acid byweight, per weight of sulphuric acid and water).

1. A method of making a membrane of sheet or hollow fibre form, themembrane comprising an aromatic ether ketone polymer, the methodcomprising the steps of: a) preparing a solution of the polymer in asolvent which is chemically inert to the polymer, the concentration ofthe polymer in the solution being at least 12%; b) forming a film of thesolution in sheet or hollow fibre form; and c) contacting the film withsulphuric acid of concentration in the range 50-80%, therebyprecipitating the membrane of said polymer.
 2. A method of making amembrane of sheet or hollow fibre form, the membrane comprising anaromatic ether ketone polymer, the method comprising the steps of: a)preparing a solution of the polymer in a solvent which is chemicallyinert to the polymer; b) forming a film of the solution in sheet orhollow fibre form; and c) contacting the film with a non-solvent,thereby precipitating the membrane of said polymer, the interval betweenthe addition of the non-solvent and the completion of the precipitationbeing at least 5 minutes.
 3. A method of making a membrane of sheet orhollow fibre form, the membrane comprising an aromatic ether ketonepolymer, the method comprising the steps of: a) preparing a solution ofthe polymer in a solvent which is chemically inert to the polymer; b)forming a film of the solution in sheet or hollow fibre form; and c)contacting the film with a non-solvent, thereby precipitating themembrane of said polymer; wherein the solvent and non-solvent areselected such that when they are mixed the temperature rises by not morethan 30° C.
 4. A method according to claim 1, wherein the film iscontacted with sulphuric acid or an alternative non-solvent therebyprecipitating the membrane of said polymer and the interval between theaddition of the sulphuric acid or other non-solvent and the completionof the precipitation is at least 5 minutes.
 5. A method according toclaim 1, wherein the solvent and sulphuric acid or other non-solvent areselected such that when they are mixed the temperature rises by not morethan 30° C.
 6. A method according to claim 1, wherein said aromaticether ketone polymer is partially crystalline.
 7. A method according toclaim 1, wherein said aromatic ether ketone polymer is a polymercomprising inter-ring ether linkages and inter-ring ketone linkagesbetween aromatic units in the polymer backbone.
 8. A method according toclaim 1, which comprises a polyetherketone homopolymer or copolymer. 9.A method according to claim 1, wherein the solvent used in thepreparation of said solution of the polymer is a strong acid.
 10. Amethod according to claim 1, wherein the solvent used in the preparationof said solution of the polymer is a substantially non-sulphonatingreagent towards the polymer.
 11. A method according to claim 1, whereina film of the solution is cast onto a plane non-porous surface.
 12. Amethod according to claim 1, wherein the concentration of the polymer insolution in the solvent is at least 16%.
 13. A method according to claim2, wherein a said non-solvent is selected from polyhydric alcohols,acetic acid and sulphuric acid.
 14. A method according to claim 2,wherein said non-solvent is acetic acid of concentration at least 70%.15. A method according to claim 1, wherein said non-solvent is sulphuricacid of concentration of at least 40% and not exceeding 80%.
 16. Amethod of making a sheet or hollow fibre membrane of an at leastpartially crystalline aromatic ether ketone polymer comprisingO-phenyl-C(O)— units and substantially free of —O-phenyl-O— or otherelectron-rich readily sulphonatable units, the method comprising thesteps of a) preparing a solution of the polymer in concentratedsulphuric acid of concentration at least 90%, the concentration of thepolymer in the solution being in the range 14-30%; b) forming a film ofthe solution in sheet or hollow fibre shape; and c) contacting the filmwith sulphuric acid of strength in the range 50-80% therebyprecipitating the sheet or hollow membrane of the polymer.
 17. A methodof preparing a polyetherketone membrane, the method including the stepsof: preparing a solution of polyetherketone in concentrated sulphuricacid; causing phase inversion by use of a non-solvent selected fromdiluted sulphuric acid, acetic acid, ethanol, methanol and glycerol. 18.A membrane of a partially crystalline aromatic ether ketone polymer, themembrane being of sheet or hollow fibre form, the membrane comprising,in cross-section, a surface layer and a base layer which is wholly orpredominantly of “sponge” cellular void structure.
 19. Use of a membraneprepared as described in claim 1 or as defined above, as a gasseparator, wherein the membrane is used without being coated by orlaminated to a different material.
 20. A gas separation apparatuscomprising a membrane prepared as described in claim 1 or being asdefined above.
 21. A membrane preparable by a method according to claim1.